[Chicken-users] srfi-18 threads question

[brandon]

Hi All,

New to this list and srfi-18 threads.  I want to make use of parameters in a thread-safe way, but I have run into problems.  It seems to me that parameters created in one thread cannot be manipulated in a persistent way from another thread. Here is some example code showing what I mean:

(use srfi-18)

 ;; mutex-protect parameter

(define (make-synchronized-parameter value)

  (let ((mutex (make-mutex))

        (param (make-parameter value)))

    (lambda args

      (mutex-lock! mutex)

      (let ((result (apply param args)))

        (mutex-unlock! mutex)

        result))))

       

(define *safe-param* (make-synchronized-parameter #f)) (define *unsafe-global* #f)

 

(define (mutate-things)

  (set! *unsafe-global* 42)

  (*safe-param* 42)

  (print "2thread> unsafe-global is "*unsafe-global*" safe-param is "(*safe-param*))

  (thread-sleep! 2) ;; make global safe for test

  (print "4thread> just before exit unsafe-global is "*unsafe-global*" safe-param is "(*safe-param*)))

 

(define (main)

  (let* ((thread (make-thread mutate-things)))

    (print "1main> before thread-start unsafe-global is "*unsafe-global*" safe-param is "(*safe-param*))

    (thread-start! thread)

    (thread-sleep! 1) ;; make global safe for test

    (print "3main> before thread-join unsafe-global is "*unsafe-global*" safe-param is "(*safe-param*))

    (thread-join! thread)

    (print "5main> after thread-join unsafe-global is "*unsafe-global*" safe-param is "(*safe-param*))))

 

(main)

 

;; actual stdout:

;; 1main> before thread-start unsafe-global is #f safe-param is #f

;; 2thread> unsafe-global is 42 safe-param is 42

;; 3main> before thread-join unsafe-global is 42 safe-param is #f

;; 4thread> just before exit unsafe-global is 42 safe-param is 42

;; 5main> after thread-join unsafe-global is 42 safe-param is #f

 

;; expected stdout for line 5:

;; 5main> after thread-join unsafe-global is 42 safe-param is 42

 

;; Why does the mutation of a simple global in a thread become visible to the main thread, but the mutation of global parameter does not?

;; What is the right way to achieve an thread-safe parameter?

Thank you!

   -Brandon